1. Field of Invention
The present invention relates to an exposure mask used in lithography in the process of manufacturing semiconductor devices, a method of fabricating the mask, and a method of manufacturing semiconductor devices.
2. Discussion of the Background
Various super-resolution techniques are being utilized in the lithography technology, since exposures based on circular illumination using conventional circular light sources are unable to obtain sufficient depths of focus. As such super-resolution techniques, a deformed illumination method and a Levenson phase shift method (Japanese Patent Application Publication No. 50811/1987) have been proposed.
In the deformed illumination method, the angle of incidence of illuminating light is selected such that only 0th-order diffracted light and one of .+-.1st-order diffracted lights from a pattern enters a projection optical system, and that 0th-order light and one of .+-.1st-order lights generally pass a target position in a pupil's plane. This method makes it possible to increase the depth of focus and increase the resolution.
However, although these techniques substantially improve the resolution and increase the depth of focus with respect to cyclic patterns such as line-and-space patterns, there is a restriction in the pattern layout in that these techniques are not effective with respect to isolated patterns. The reason for this is that although, in the deformed illumination method and the Levenson phase shift method, a state of two-beam-flux interference is produced by making use of diffracted light generated due to the cycle of the pattern so as to increase the depth of focus, it is difficult to produce the state of two-beam-flux interference with respect to isolated patterns since the cycle of the pattern is large.
FIGS. 12A and 12B are schematic diagrams for explaining the theory of diffraction in the reduction exposure technology. FIG. 12A is a schematic diagram in a case where a mask having a small pattern pitch is used, and FIG. 12B is a schematic diagram in a case where a mask having a large pattern pitch is used.
In FIG. 12A, reference numeral 57 denotes light for exposure which is emitted from a light source (not shown) of a stepper; 58, a mask with a small pattern pitch and having light-shielding portions 63 and transparent portions 64; 59, a lens on which the light transmitted through the mask is made incident after being separated; 60, the pupil's plane of the lens; and 62, a wafer surface which constitutes the plane of projection.
Referring to FIG. 12A, the light 57 which has been transmitted through the mask 58 having the small pattern pitch is separated into 0th-order diffracted light 52 which advances straightly along the optical axis, .+-.1st-order diffracted light 53 having an angle of .theta. (sin.theta.=.lambda./P; .lambda., wavelength of exposing light; P, pattern pitch) with respect to the optical axis, and -1st-order diffracted light 54, is made incident upon the pupil's plane 60 of the lens 59, and forms an image on the wafer surface, thereby forming an image of the pattern of the mask 58.
Meanwhile, FIG. 12B shows a case where the light 57 is made incident upon a mask 61 with a large pattern pitch and having the light-shielding portions 63 and the transparent portions 64. In FIG. 12B, the light 57 which has been transmitted through the mask 61 having the large pattern pitch is separated into the 0th-order diffracted light 52 which advances straightly along the optical axis, the .+-.1st-order diffracted light 53 having the angle of .theta. with respect to the optical axis, the -1st-order diffracted light 54, +2nd-order diffracted light 55, -2nd-order diffracted light 51, +3rd-order diffracted light 56, -3rd-order diffracted light 50, is made incident upon the pupil's plane 60 of the lens 59, and forms an image on the wafer surface, thereby forming an image of the pattern of the mask 61.
In FIGS. 12A and 12B, only the diffracted light which has been transmitted through the inner side of the pupil's plane 60 of the lens 59 contributes to the formation of an image on the wafer surface 62. For this reason, in FIG. 12B, since the pattern pitch of the mask is large, the angle .theta. with respect to the optical axis becomes small, with the result that the .+-.1st-order diffracted light, the .+-.2nd-order diffracted light, the .+-.3rd-order diffracted light, . . . are made incident upon the inner side of the pupil's plane 60 of the lens 59, thereby causing multiple-beam-flux interference to occur.
FIGS. 13A and 13B are schematic diagrams illustrating the states of diffracted light formed in the pupil's plane of a projection lens system in cases where the masks shown in FIGS. 12A and 12B are used. FIG. 13A shows the diffracted light in the pupil's plane after being transmitted through the mask 58 having the small pattern pitch, and FIG. 13B shows the diffracted light in the pupil's plane after being transmitted through the mask 61 having the large pattern pitch.
In FIGS. 13A and 13B, reference numeral 60 denotes the pupil's plane of the lens; 52, the 0th-order diffracted light; 53, the +1st-order diffracted light; 54, the .+-.1st-order diffracted light; 55, the +2nd-order diffracted light; 51, the -2nd-order diffracted light; 56, the +3rd-order diffracted light; and 50, the -3rd-order diffracted light.
In FIG. 13B, since the pitch of the pattern formed on the mask is large, the angle .theta. of incidence upon the pupil's plane is small, and the interval of the diffracted light becomes small, so that even higher-order diffracted light enters the pupil's plane 60. Consequently, multiple-beam-flux interference occurs in the case of an isolated pattern with the large pattern pitch, and the depth of focus decreases as compared with the continuous pattern with the small pitch shown in FIG. 13A.
Here, referring to FIGS. 14A and 14B, a description will be given of the cause of the decrease in the depth of focus due to the occurrence of the multiple-beam-flux interference. FIGS. 14A and 14B are schematic diagrams for explaining image formation due to various types of interference. FIG. 14A shows image formation due to the multiple-beam-flux interference by taking three-beam-flux interference as an example, and FIG. 14B shows image formation due to two-beam-flux interference.
In the three-beam-flux interference shown in FIG. 14A, since the phase of .+-.1st-order light with 0th-order light as a reference changes with defocusing, the contrast of the image declines. This is more noticeable with multiple-beam-flux interference of three beam fluxes or more. Meanwhile, in the two-beam-flux interference shown in FIG. 14B, troughs (mutually weakening portions) of two waves and ridges (mutually strengthening portions) thereof occur at the same positions, respectively, the phase difference between them does not occur even during defocusing, so that the contrast of the image does not decline.
Thus, in the multiple-beam-flux interference, the phase relationship changes when the plane of projection is offset from the plane of the best focus in the defocusing direction, making it impossible to maintain the contrast of the image. Therefore, there occurs the problem that the depth of focus decreases.
As a method for overcoming this problem, Unexamined Japanese Patent Publication No. 6-275492, for instance, proposes a method for increasing the depth of focus of an isolated pattern in a case where a continuous pattern and an isolated pattern are present on the same mask in a mixed form.
Here, a description will be given of the method disclosed in Unexamined Japanese Patent No. 6-275492 for increasing the depth of focus of an isolated pattern in the case where a continuous pattern and an isolated pattern are present on the same mask in a mixed form.
FIGS. 15A to 15C are plan views of mask patterns illustrating the conventional method for increasing the depth of focus of isolated patterns.
First, FIG. 15A is a plan view illustrating a line-and-space pattern. FIG. 15B is a plan view illustrating an isolated pattern 69 and an auxiliary pattern 70 which are delineated on the same mask as that of FIG. 15A. FIG. 15C is a plan view illustrating an inverted pattern of the auxiliary pattern shown in FIG. 15B.
In FIGS. 15A and 15B, a line-and-space pattern 68 as well as the isolated pattern 69 and its auxiliary pattern 70 are formed on a mask 67. In FIG. 15C, an inverted pattern 72 with Cr removed for inverting the auxiliary pattern 70 is formed on a mask 71 which is a layer separate from the mask 67.
First, by using the mask 67, reduction projection exposure is effected by oblique incidence illumination from a four-split light source, and the line-and-space pattern 68, the isolated pattern 69, and the auxiliary pattern 70 are transferred onto a posi-type photoresist on a wafer. Subsequently, by using the mask 71, reduction projection exposure is effected in a similar manner, thereby causing the transferred pattern of the auxiliary pattern 70 formed on both sides of the isolated pattern 69 to disappear.
With the conventional method, however, the auxiliary patterns provided on both sides of the isolated pattern for increasing the depth of focus of the isolated pattern are resolved on the wafer. For this reason, the mask for causing the auxiliary pattern to disappear by the use of the inverted pattern is required, and a step for causing the auxiliary pattern to disappear by using the mask is required, with the result that the number of fabrication steps increases. In addition, there is a problem in that in a case where an alignment error has occurred during pattern exposure, the function of causing the auxiliary pattern to disappear degrades.